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Centralize virtualization at the switch

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Virtualization applications
Storage virtualization is employed to solve specific business problems, such as simplified management, cost reduction or a need for nondisruptive data migration between arrays. "No one buys storage virtualization for virtualization per se, but to fill a very specific need; this is very different from server virtualization," says Robert Infantino, Incipient's senior VP of marketing and alliances.

Data migration
Data migration is by far the leading reason users deploy storage virtualization. As the virtualization layer controls the virtual to physical mapping, a virtualization product is able to forward storage requests to the correct physical device even while data is migrated and spread between a source and destination device. All virtualization products, regardless of their underlying architecture, support data migration. Performance of data migration services, however, will vary among the different virtualization architectures; Hitachi claims to have a performance advantage because its virtualization resides within the storage controller.

Provisioning and volume management
Provisioning of virtualized volumes is a core service in all virtualization products. Besides simplified storage management, centralized provisioning through the virtualization software enables higher storage utilization because storage is provisioned

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more granularly. Similarly, the volume management feature in virtualization products is instrumental in increasing storage utilization. By aggregating multiple physical disks to present them as a single large disk, and disaggregating large disks to present them as multiple smaller volumes, storage can be managed more effectively.

Thin provisioning
Thin provisioning is currently not available in fabric-based virtualization products. The lack of cache and the stateless nature of fabric-based virtualization make it more difficult to implement. Hitachi was the first vendor to offer thin provisioning in its USP V product, offering the first 10TB of thin-provisioned storage for free. IBM added thin provisioning in the recently released SVC 4.3 along with space-efficient snapshots and virtual disk mirroring. LSI has committed to releasing thin provisioning later this year and Incipient has it on its roadmap; EMC and Fujitsu have no intention at this point to offer thin provisioning in their virtualization products. "We currently support thin provisioning in our arrays and haven't decided if and when we will support it in Invista," says Doc D'Errico, VP of the infrastructure software group at EMC.

Snapshots
Snapshots or clones are supported by all virtualization vendors except Fujitsu. However, space-efficient snapshots (snapshots that require disk space for changes between snapshots) are currently supported only by IBM's SVC, Hitachi's USP V and LSI's SVM. Akin to thin provisioning, space-efficient snapshots are more difficult to support in fabric-based virtualization products. While EMC's Invista supports only full-copy clones, users have the option to deploy RecoverPoint in addition to Invista to take advantage of space-efficient snapshots. LSI's virtualization proves that the app challenges of fabric-based virtualization can be overcome; not only does LSI offer space-efficient snapshots, its SVM product supports consistency groups with snapshots, as well as copy and mirroring, which enables entire apps to be snapped or copied at once and then recovered within minutes.

The ability to scale
Performance and scalability are the two main benefits of fabric-based virtualization. Switch-based virtualization products can be scaled vertically if the switch vendor supports multiple intelligent fabric modules in a single switch and horizontally by deploying additional intelligent switches. For example, storage architects can populate a single Cisco MDS switch or director with multiple intelligent line cards, or intelligent line cards can be inserted into multiple switches. Scaling is also achieved by deploying newer generations of intelligent line cards. "Cisco is already shipping the MSM-18/4, which is Cisco's second-generation intelligent card; in the fourth quarter of this year, Cisco will ship its third-generation intelligent line cards," says Rajeev Bhardwaj, director of product management at Cisco's Data Center Business Group.

There's currently no perfect virtualization product and users need to carefully weigh the product that best fits their environment's requirements. For companies that have standardized on Hitachi storage, Hitachi's array-based virtualization is likely at the top of their list. Companies with a Cisco- or Brocade-based SAN should consider Invista or Incipient; they'll also need to weigh the performance and scalability benefits against some of the challenges of these products such as their relatively high cost, complexity and feature constraints. LSI deserves consideration as it's currently the only vendor offering a virtualization product that combines the simplicity and rich feature set of an in-band appliance like IBM's SVC with the benefits of fabric-based virtualization.

This was first published in September 2008

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